ICU-22843 Disambiguate UnicodeString::readOnlyAlias() for MSVC

[roubert]

Both std::u16string_view and std::wstring_view are possible matches for UnicodeString as a template parameter, but adding an explicit overload avoids both having to make that choice and taking the detour through creating any string view at all.

Checklist

• Required: Issue filed: https://unicode-org.atlassian.net/browse/ICU-22843
• Required: The PR title must be prefixed with a JIRA Issue number.
• Required: The PR description must include the link to the Jira Issue, for example by completing the URL in the first checklist item
• Required: Each commit message must be prefixed with a JIRA Issue number.
• Issue accepted (done by Technical Committee after discussion)
• Tests included, if applicable
• API docs and/or User Guide docs changed or added, if applicable

ALLOW_MANY_COMMITS=true

[roubert]

The test case in the first commit fails like this with MSVC:

https://github.com/unicode-org/icu/actions/runs/10816568529/job/30008039132?pr=3150

The fix in the second commit makes the test case work also with MSVC.

[markusicu]

@richgillam FYI

[markusicu]

The test case in the first commit fails like this with MSVC:

good catch!

[jira-pull-request-webhook]

Notice: the branch changed across the force-push!

• icu4c/source/common/unicode/unistr.h is different

View Diff Across Force-Push

~ Your Friendly Jira-GitHub PR Checker Bot

[jira-pull-request-webhook]

Notice: the branch changed across the force-push!

• icu4c/source/common/unicode/unistr.h is different

View Diff Across Force-Push

~ Your Friendly Jira-GitHub PR Checker Bot